In recent years, a fuel cell is attracting attention as a clean energy source. Examples of the fuel cell are a molten carbonate fuel cell, an alkali aqueous solution fuel cell, a phosphoric acid fuel cell, and a polymer electrolyte fuel cell. Since an operating temperature of each of the phosphoric acid fuel cell and the polymer electrolyte fuel cell is lower than that of each of the other fuel cells, each of the phosphoric acid fuel cell and the polymer electrolyte fuel cell is often used as the fuel cell constituting a fuel cell system. Especially, since the operating temperature of the polymer electrolyte fuel cell is low, the polymer electrolyte fuel cell is preferable as the fuel cell of a domestic fuel cell system, such as a cogeneration system.
The polymer electrolyte fuel cell is configured by stacking and fastening a plurality of cells, each including: a membrane-electrode assembly constituted by an anode, a cathode, and a polymer electrolyte membrane; an anode separator; and a cathode separator. The anode is constituted by an anode catalyst layer and an anode gas diffusion layer. The cathode is constituted by a cathode catalyst layer and a cathode gas diffusion layer. The polymer electrolyte fuel cell generates electricity and heat by a cell reaction between a fuel gas supplied to the anode and an oxidizing gas supplied to the cathode.
It is known that in this case, in order to suppress the deterioration of the polymer electrolyte membrane to extend the life of the fuel cell, the fuel cell is operated by being supplied with a reactant gas having a dew point that is equal to a temperature of the cell (full humidification operation) (see Patent Document 1).
The anode catalyst layer of the polymer electrolyte fuel cell may deteriorate by being poisoned by carbon monoxide slightly contained in the supplied fuel gas. Here, proposed is that the anode catalyst layer is formed to include two layers, and a CO-resistant catalyst is used as a catalyst of one of the two layers (see Patent Document 2). With this configuration, the poisoning of the anode is suppressed even if carbon monoxide remains in the fuel gas supplied to the anode.
Moreover, known is a fuel cell in which in order to suppress the deterioration of the electrolyte membrane by hydrogen peroxide, a material having a hydrogen peroxide decomposing ability is included in at least one of the electrolyte membrane, the anode, and a portion between the electrolyte membrane and the anode (see Patent Document 3 for example).
Moreover, known is a polymer electrolyte membrane-electrode assembly in which a second catalyst layer formed by a catalyst, an electrically conductive carrier, and an ionomer is disposed between a relatively dry region in a planar direction of the MEA and the polymer electrolyte (see Patent Document 4 for example). In accordance with the polymer electrolyte membrane-electrode assembly, the generation of hydrogen peroxide by cross leakage is suppressed, and the decomposition and deterioration of the electrolyte membrane by hydrogen peroxide can be therefore effectively suppressed.
Moreover, known is a membrane-electrode assembly which is for use in a solid polymer electrolyte fuel cell and in which in order to improve a water retention property, a layer making a contact angle of 10 degrees or less with water and containing an inorganic compound is disposed between the polymer electrolyte and the catalyst layer of at least one of the cathode and the anode (see Patent Document 5).
Moreover, known is a fuel cell in which in order to suppress a crossover of hydrogen and methanol, a thin layer formed by a catalyst, an oxide, and a polymer electrolyte is disposed on an interface between the cathode and the polymer electrolyte (see Patent Document 6 for example).
Moreover, known is a solid polymer electrolyte composition in which in order to improve the water retention property and suppress the cross leakage of hydrogen, fine particles of precious metal and metal oxide are dispersed in the polymer electrolyte (see Patent Document 7 for example).    Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2006-210334    Patent Document 2: Published Japanese Translation of PCT Application No. 2002-532833    Patent Document 3: Japanese Laid-Open Patent Application Publication No. 2006-244782    Patent Document 4: Japanese Laid-Open Patent Application Publication No. 2006-338941    Patent Document 5: Japanese Laid-Open Patent Application Publication No. 2003-288915    Patent Document 6: Japanese Laid-Open Patent Application Publication No. 2003-086192    Patent Document 7: Japanese Patent No. 3375200